Sixteen people were treated for minor injuries at the hospital in Lixouri, the island's second largest town and the one closest to the epicenter, hospital director Nikos Markatos said. Local residents calling in to early morning radio and television news shows reported damaged roads and cuts to the power and water supply in some areas.
Kefalonia Mayor Alexandros Parisis said the port in Lixouri had been damaged, and images after first light showed part of the pier had crumbled into the sea. Problems were also reported on the road between Lixouri and the main part of the island.
Earthquakes have been rattling this island constantly for the past week, after a 5.9-magnitude temblor struck the area on Jan. 26, damaging homes and slightly injuring seven people. Since then, thousands of residents have been spending nights with relatives or in ships sent for that reason.
Schools on the island have been shut for the past week, and had not been scheduled to reopen until Wednesday, said Deputy Mayor Evangelos Kekatos.
Authorities urged the islanders to remain calm and not approach any buildings that appeared damaged.
An eight-member rescue team with a sniffer dog was heading to the island as a precautionary measure, the fire department said, while Public Order Minister Nikos Dendias and the ministry's secretary general were also heading to Kefalonia to coordinate civil protection efforts, authorities said.
The Athens Geodynamic Institute registered the pre-dawn quake, which struck just after 5 a.m. local time, with a magnitude of 5.7 and an epicenter 12 kilometers (7.5 miles) northwest of the island's capital of Argostoli. The U.S. Geological Survey registered a 6.1-magnitude. It is common for institutes to register different figures.
The intense seismic activity has reawakened memories of the devastating 1953 quakes on Kefalonia and neighboring Zakinthos, when a 7.2-magnitude temblor struck three days after a 6.4 quake, killing hundreds, injuring thousands and destroying nearly all the buildings on the islands.
Seismologists said more aftershocks were to be expected on the Ionian island, which lies in a highly seismically active region. Several registered in the initial hours after Monday's quake.
"The whole area has been activated ... we are all on alert," said Maria Sahpazi, head of the Geodynamic Institute, adding that Monday morning's quake appeared to be a strong aftershock of the Jan. 26 temblor.
"We expect more aftershocks, which will be of this size or smaller," she said.
Monday's quake was felt across parts of the western Greek mainland and as far away as the Greek capital, Athens, nearly 300 kilometers (186 miles) to the west.
Kefalonia's mayor and seismologists urged people on the island to leave their houses temporarily.
Christos Kyrakatos, a resident of one of Kefalonia's villages, told Greek television that at least one person in his village had been taken away by ambulance after being injured by falling objects in his home. The power and water supply to the area had been cut, and residents who had fled their homes also had to contend with intense bad weather, with strong rain and low temperatures.